Re: What is sodium polyacrylate and when was it invented?

Dear John,
Sodium polyacrylate is a polyelectrolyte, i.e. a polymer (long-chain 
molecule material) carrying "salt groups" incorporated along its chain. The 
polymer itself is the anion, and the counterion is sodium ion (Na+). The 
negatively charged groups are thus fixed on the chain, the sodium ions are 
attracted by electrostatic forces, but relatively mobile. This allows them 
to be hydrated, i.e. surrounded by water molecules. This leads us to the 
application which the madsci administrator thinks you are most interested 
in: superabsorbent materials like diapers etc. I had to look it up myself. 
The oldest patent I found was by R.T. Elias and Dennis C. Holtman. The 
summary reads as follows:

TI   Thin-skin stabilization of pads of fluffed pulp
IN   Elias, Robert T.; Holtman, Dennis C.
PA   Johnson and Johnson Baby Products Co., USA
SO   U.S., 4 pp. Cont.-in-part of U.S. Ser. No. 943,277, abandoned.
     CODEN: USXXAM
PI   US 4233345  1980 11 11
AI   US 78-943277  19780918
DT   Patent
LA   English
IC   B05D003-00; B05D003-12
NCL  427325000
CC   43-3 (Cellulose, Lignin, Paper, and Other Wood Products)
AB   High-loft, low-d., air-laid pads of short fibers, e.g. for use in 
sanitary napkins or diapers,
     are stabilized by spraying their surface with a soln. of coagulant and 
then with a resin
     dispersion, the penetration of which is limited by instant 
coagulation.  Thus, 26-g pads of
     air-layed wood pulp fibers (265 g/m2) are sprayed with 5 mL 5% NaOH/m2 
and then with 45 mL 1%
     poly(acrylic acid)  [9003-01-4] (Carbopol 940, mol. wt. 4 .times. 
106)/m2, which gels
     immediately to form a thin skin stabilizing the pad without loss of 
bulk, loft, or absorbance.
ST   pulp pad stabilization; coagulant stabilizer pulp pad; fiber cellulose 
pad stabilization;
     acrylic acid polymer stabilizer pad; diaper pulp stabilizer
IT   Diapers
        (cellulose fiber pads for, stabilization of, by poly(acrylic acid))
IT   Pulp, cellulose
        (fiber pads, stabilization of, penetration prevention in)
IT   Surgical dressings and goods
        (sanitary napkins, cellulose fiber pads for, stabilization of, by 
poly(acrylic acid))
IT   1310-73-2, uses and miscellaneous
     RL: USES (Uses)
        (coagulants, for prevention of penetration of poly(acrylic acid) in 
stabilization of
        cellulose fiber pads)
IT   9003-01-4
     RL: USES (Uses)
        (stabilization by, of cellulose fiber pads, penetration prevention 
in)

As you can see, sodium hydroxide is first applied to the fibers, then 
polyacrylic acid. The result is sodium polyacrylate precipitated on the 
fibers. Now the essential trick is the following: if you take a classical 
water-loving (hydrophilic) absorbent like cotton, its volume shrinks a lot 
when you soak it in water, because the surface tension of the water drives 
the system to minimize water interface area. A superabsorbent, on the other 
hand, due to the polyelectrolyte which glues the fibers together in a 
crosslinked structure, is BUTTRESSED AGAINST SHRINKAGE (the fibers cannot 
move relative to each other). The WHOLE VOLUME spanned by the original 
"fluffy" fibers can thus be filled by water. In terms of weight, this is a 
multiple of the dry fiber weight. 
Considering that this patent was granted in 1980, the idea really seems to 
have filled a need..
Best regards
Werner Sieber